Antenna comprising restraining means for resilient support members

ABSTRACT

THE SUBREFLECTOR OF A CASSEGRAINIAN OR GREGORIAN ANTENNA SYSTEM IS MOUNTED BY POSITIONING A SINGLE LATERALLY BENT LEG SUPPORT MEMBER BETWEEN THE MAIN REFLECTOR DISH AND THE SUBREFLECTOR. GUYING WIRES ARE USED TO POSITION THE SUBREFLECTOR IN FRONT OF THE FEED SOURCE AND TO PRELOAD THE LEG SUPPORT MEMBER TO PROVIDE MINIMUM DEFLECTION OF THE SUPPORT MEMBER AND CONSEQUENTLY MINIMUM MOVEMENT OF THE SUBREFLECTOR WITH RESPECT TO THE MAIN REFLECTOR UPON THE APPLICATION OF EXTERNAL FORCES TO THE SUBREFLECTOR.

Jan. 5, 1971 TSZRTES 3,553,731

ANTENNA COMPRI G RESTR ING MEANSFOR RE IE SUP T MEMBERS led May 1967 730/1741 .52/7'735 BY W g M AITGIIIEY United States Patent 3,553,731 ANTENNA COMPRISING RESTRAINING MEANS FOR RESILIENT SUPPORT MEMBERS Thomas Szirtes, Montreal, Quebec, Canada, assignor to RCA Corporation, a corporation of Delaware Filed May 15, 1967, Ser. No. 638,522 Int. Cl. H0111 19/14 US. Cl. 343-781 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an antenna structure and, more particularly, to the use of a single legged support structure prestressed to form a support for the subreflector of an antenna system such as a Cassegrainian or Gregorian antenna system.

The term Cassegrainian Antenna System refers to that type of antenna system wherein there is provided a main reflector dish with a hole or feed in the center thereof. A second (hyperboloid) reflector, subreflector, is located along the optical axis of the main reflector between the main reflector and the focus point so that a received signal is doubly reflected and then fed through the feed at the center of the main reflector. Cassegrainian antenna systems including a hyperboloid subreflector are well known and extensively used in the state of the art.

The term Gregorian Antenna System refers to that type of antenna system wherein there is provided a sub reflector that is ellipitcal and located beyond the focus of a paraboloidal main dish. The normally used means for mounting the subreflector to the premary reflector dish in either type of antenna is by the use of three or four large structural support legs providing what is more commonly known as a tripod or quadrupod structure. Each of these three or four support legs when used for large antenna structures is substantially large in itself and is heavy in weight. The use of such structural support legs decreases the surface accuracy, deteriorates the natural frequency charactistics of the main reflector dish, and increases the geometrical blockage which increases the noise temperature and decreases the gain. This problem becomes particularly acute when constructing a larger sited for example an 85 ft. diameter, 36 ft. focal length) Cassegrainian or Gregorian antenna system where the weight of the subreflector may be up to 650 pounds, requiring large and heavy support legs. In such a structure the weight of a four-legged support structure quadr-upod may be as much as 7,800 pounds. Also, because of the complexity of the four-legged structure used, the manufacture and erection of the supporting legs is costly.

It is an object of this invention to provide an improved support structure for antennas that is relatively light weight and hassuperior rigidity, substantially reduced RF blockage and improved vibration characteristics.

It is a further object of this invention to provide a single, simplified structure, which is preloaded using a restraining means, for support of a subreflector in an antenna system.

Briefly, in accordance with this invention there is provided a single elongated support member laterally deflected or bent. One end is coupled to the back-frame of the main reflector dish of an antenna. The other, opposite end extends generally in a direction away from the main reflector dish and is connected to a member, for example, a subreflector, to be supported and spaced from the main reflector dish. A plurality of restraining members such as wires are coupled to the support member and anchored to the back-frame of the main reflector dish. The restraining members are adjusted and arranged to preload the support member to provide minimum deflection of the support member with respect to the main reflector upon the application of external forces to the subreflector and to position the supported and spaced member from the main reflector dish in the desired position relative to the main reflector dish.

A more detailed description follows in conjunction with the single figure of the accompanying drawing of a Cassegrainian antenna system in accordance with one embodiment of this invention.

The single figure presented shows a perspective view of a Cassegrianian antenna system and a support structure in accordance with this invention. The antenna system comprises a parabolic dish .10 which may be, for example, approximately ft. in diameter with the focal length of the antenna 36 ft. A feed horn 11 having an aperture 12 is located at the center of the vertex of the parabolic main reflector dish 10. A hyperbolic subreflector 13 is mounted at the free end of a single, spring-loaded support member 14 by a suitable clamping assembly 16. The subreflector 13 is mounted symmetrically with the main reflector dish 10 and is located between the main reflector dish 10 and the focus point. The subreflector 13 may weigh, for example, about 350 pounds. The single leg support member in accordance with one embodiment of this invention is an elongated leg member that is laterally deflected or bent in the form of a semi S shaped or hook shaped spring-like member. The laterally deflected leg support member 14 is anchored at one end to main reflector dish 10 and is preloaded by six guy wires 21 through 26 anchored to the back frame of the main reflector dish 10. The leg support member 14 may in the above example of an 85 ft. diameter Cassegrainian antenna system be a conical hollow tube 12 inches in diameter at the bottom and 7.5 inches in diameter at the top. The leg support member 14 is seam welded and is laterally deflected or bent into the semi S shape as shown in the accompanying figure. The tubular material may be a high strength galvanized carbon steel.

A crosspiece 20 is coupled to the top free end of leg support member 14. The guy wires 21 and 22 are coupled at one end to the end points 37 and 38, respectively of crosspiece 20 and at their opposite ends are anchored to the back-frame of the main reflector dish 10 at points 41 and 42 through turnbuckles 31 and 32. The guy wires 21 and 22 provide a stress in a first direction away from the lateral deflection or bend of support member 14. The guy wires 23 and 24 are coupled at one end to the end points 37 and 38 of crosspiece 20, respectively, and at their opposite ends are anchored at points 43 and 44 on the back-frame of the main reflector dish 10 through turnbuckles 33 and 34. The guy wires 23 and 24 provide a stress in an* opposite direction with respect to guy wires 21 and 22 and provide a stress away from the lateral deflection of the support member 14. The guy Wires 25 and 26 are coupled at one end to a clamp 39 positioned along the middle portion of the lateral deflection of the semi S shaped leg support member 14 as shown in the figure. The guy wires 25 and 26 are anchored to points 45 and 46 on the back-frame of the main reflector dish 10 through turnbuckles 35 and 36. The guy wires 25 and 26 provide a stress in the direction of the lateral deflection of support member 14 opposed to the direction of the stress provided by guy wires 21 through 24.

If support member 14 having a stiffness factor K (defined as a force per unit deflection constant) is sub jected to an initial load P for example, it can be shown that in order to provide unit deformation on the top of the initial deflection produced by P a force P=K+P is necessary. By increasing P one can make the support structure as stiff as necessary. By a large P the stresses will also increase so a proper compromise has to be made. By the proper adjustment of the turnbuckles 31 through 36 shown in the figure, the subreflector 13 located near the free end of the support member 14 is positioned in the optimum position in front of the feed horn 11 and symmetrical to the main reflector dish 10. The laterally bent elongated support member 14 is preloaded by the guy wires 21 through 26 by proper adjustment of the turnbuckles shown in the figure. By the use of the laterally deflected support member 14 as shown in the figure in combination with the guy Wires to preload the support structure, a support structure having high stress, low volume and small deflection is provided. By the use of the structure shown in the figure, a support structure for an antenna having the dimensions cited above can be provided weighing about 1900 pounds to support a 350 pound subreflector in an 85 ft. diameter, 36 ft. focal length Cassegrainian antenna system. In addition to the other advantages gained by the use of the described structure such as light weight, superior rigidity, and improved vibration characteristics, there is also a reduction of RF blockage due to the support and consequently an increased gain and decrease of noise temperature as compared to previously known structures.

What is claimed is:

1. In an antenna comprising a reflector and a member supported and spaced from said reflector, the combination comprising:

a single resilient elongated support member having a lateral bend along the length thereof and having a first end coupled to said reflector and extending in a direction away from said reflector,

said supported member being coupled to and near the opposite end of said support member,

prestressing restraining means coupled to said support member and anchored to said reflector,

said restraining means being arranged and adjusted to provide suflicient preloading to said support member to provide minimum deflection of said support member with respect to said reflector upon the application of external forces to said support member and to position said support member in the desired spaced position from said reflector.

2. In an antenna system having a main reflector dish,

a feed source and a subreflector, the combination comprising:

a single resilient elongated support member having a lateral bend along the length thereof and having one end coupled to the back-frame of said main reflector dish with said support member extending generally away from said main reflector dish,

said subreflector being coupled to and near the opposite, free end of said support member,

prestressing restraining means coupled to said support member and anchored to the back-frame of said main reflector dish in a manner to provide sufficient preloading to said support member to provide minimum movement of said subreflector with respect to said main reflector dish upon the application of external forces to said subreflector and to position said subreflector in the desired position along the optical axis relative to said main reflector dish and opposite said feed source.

3. The combination as claimed in claim 2 wherein said restraining means includes a plurality of wires anchored in dilferent directions to said main reflector to provide preloading to said support member.

4. The combination as claimed in claim 3 wherein said support member is an elongated member having a lateral bend within the middle portion of said support member.

5. In an antenna system having a main parabolic reflector dish, a feed source and a subreflector dish, the combination comprising:

a single laterally deflected elongated support member having one end coupled to the back-frame of said main reflector dish with said support member extending away from said main reflector dish, said subreflector being coupled to and near the opposite, free end of said support member,

a first wire means coupled to said free end of said support member and anchored to the back-frame of said main reflector dish in a first direction away from said lateral deflection,

a second Wire means coupled to said free end of said support member and anchored to the back-frame of said main reflector dish in a second direction away from said first direction and away from said lateral deflection,

a third wire means coupled approximately to the middle of the lateral deflected portion of said support member and anchored to the back-frame of said main reflector dish in the direction of said lateral deflection and away from said first and second directions, said wire means being adjusted and arranged to provide suflicient preloading to said support member to provide minimum deflection of said support member and consequently minimum movement of said subreflector with respect to said main reflector upon the application of external forces to said subreflector and to position said subreflector in the desired position along the optical axis of said main reflection dish and opposite said feed source.

6. The combination as claimed in claim 5 wherein said support member is an elongated conical tubular member formed in a semi S shape and wherein said third wire means is coupled to the middle portion of said semi S shape.

7. An antenna system having a main reflector, a feed source an da subreflector, the combination comprising:

a single resilient elongated support member having a lateral bend along the length thereof and having one end coupled to said main reflector with said support member extending generally away from said main reflector, said subreflector being coupled to and near the opposite free end of said support member,

a first guying means coupled between said free end of said support member and said main reflector and extending in a first direction away from said lateral bend for prestressing said support member,

a second guying means coupled between approximately the middle portion of the lateral deflection of said suppoit member and said main reflector and extending in the general direction of said lateral deflection and opposed to said first direction for prestressing said support member, said first and second guying means being adjusted and arranged to provide minimum deflection of said support member and consequently minimum movement of said subreflector with respect to said main reflector upon the applica- 6 References Cited UNITED STATES PATENTS 2,540,518 2/1951 Gluyas 343-840X 3,407,404- 10/1968 Cook et a1. 343781C 5 FOREIGN PATENTS 1,064,124 8/1959 Germany 343840 PAUL L. GENSLER, Primary Examiner tion of external forces to said subreflector in the 10 desired position along the optical axis relative to said main reflector opposite said feed source.

U.S. Cl. X.R. 

